A massive manhunt is under way for a suspect seen with two supposed suicide bombers shortly before they struck Brussels' airport in the first of two attacks that also hit the city's metro, killing 31 people and wounding 261.

Belgian Federal Prosecutor Frederic Van Leeuw said on Wednesday that Brahim el-Bakraoui was part of the twin suicide bombings at the Zaventem airport, while his brother, Khalid, blew himself up on a metro carriage at Maelbeek station.

A CCTV image from the Brussels Airport shows the suspects of the Brussels airport attack [Reuters]

Brahim, pictured in the middle of a surveillance photo at the airport, had been identified using fingerprint records, Van Leeuw said.

The second airport suicide bomber, on the left of the picture, has not been identified. The third suspect, who left the airport before the explosions, is also not yet identified and remained on the run, Van Leeuw said.

The prosecutor also confirmed that police had carried out raids following Tuesday morning's attacks, including at an address associated with the suspects after a tip-off by a taxi driver who drove the attackers to the airport.

During a house search in the Brussels neighbourhood of Schaerbeek, investigators found 15 kilos of explosives, detonators and chemical products, Van Leeuw said.

Police also found a computer in a rubbish container in the street that included a note by Brahim, the airport bomber, in which he said he felt increasingly unsafe and feared landing in prison.

"Always on the run, not knowing what to do any more, being looked for everywhere, not being safe any longer," the prosecutor quoted Brahim's note as saying.

Van Leeuw said that one person arrested in one of the raids remained in custody on Wednesday and is being questioned.

The prosecutor spoke shortly after crowds of mourners gathered outside Maelbeek station, one of the targets of the attacks, to observe a minute of silence.

The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) claimed responsibility for the attacks.

"This is a day of tragedy, a black day," Belgian Prime Minister Charles Michel said, describing the bombings as the "deadliest attacks we have ever seen in Belgium".

But as Belgium began three days of national mourning on Wednesday, he insisted that the country would not be cowed by the "blind, violent and cowardly" attacks.

People gather and light candles at the Place de la Bourse during a vigil to pay tribute to the victims of the attacks [EPA]

Belgian authorities released pictures of two of the suspects pushing trolleys with their bombs through the terminal and said that they were "actively searching" for a third man whose explosives did not go off.

Police helicopters hovered over the city late into the night on Tuesday and raids were carried out across Belgium, prosecutors said, adding that a bomb, an ISIL flag and chemicals had been found in one apartment.

Police were going door-to-door throughout Brussels searching for suspects or others planning attacks. The interior minister said 600 additional police were deployed.